RE: Assist assistance needed

[Henry Harper <hah@srv.net>]

Phil -r- Judy said:

>OK, I admitted my ignorance in regard to brake assist, but no one's come
>forward yet to offer enlightenment. Hence, some more questions:
...
>(2) As an aside, how _does_ vacuum assist work? Not many psi of vacuum are
>available from the manifold, so how is this translated into positive
>assist? Must be a _big_ membrane involved.
...[other stuff I am not helping with]

Near the nice diagram on p. 181 of the December 97 _Car & Driver_, 
in the article about assisted braking systems:
*How normal power brakes work*
A conventional power-brake system uses a vacuum booster to multiply 
the force that the driver is applying to the brake pedal. When the driver's 
foot is off the pedal, the vacuum port in the brake booster is open, admitting 
vacuum from the engine's intake manifold to the pressure chamber within 
the brake booster. Since this vacuum is always applied to the vacuum 
chamber [on the other side of the diaphragm], there is no movement of the 
diaphragm. When the driver presses on the brake pedal, a bell-shaped valve 
within the brake booster moves, closing the vacuum port and opening the 
atmospheric port, which admits atmospheric pressure to the pressure 
chamber. The resulting pressure differential moves the diaphragm, adding to 
the force that the driver's foot is applying to the brake master cylinder. As the 
diaphragm moves, it closes off the atmospheric port, which keeps the booster's 
efforts proportional to the driver's pressure on the brake pedal. Hydraulic 
boosters use a generally similar approach, substituting hydraulic pressure for 
manifold vacuum.

Last sentence qualifies as Audi content, right? Also GM 6.2L diesel content, 
but there's no mailing list for that...cause they're not all that fun to drive.

Ok, here's some genuine Audi content. My indicated OAT seems to be six 
degrees low compared to a local bank thermometer (GTI is four degrees low). 
Took my first on-interstate trip tonight, and it was smoooth. The speedo needle 
was optimistic compared to the MFA display, so visions of 52ohm resistors were 
dancing in my head. But a timed mile at 71mph indicated was 51 seconds (as 
close as I could tell on my non-stopwatch-equipped wristwatch) which is darn 
close to 71mph. Maybe I need a GPS for Xmas? Mpg also settled out at around 
24.5 indicated for a 25-mile stretch of 71-mph-indicated-cruise, which I think is 
even pessimistic. Road trips will be fine, but inertia is just not your friend around 
town.

Got sick of bouncing and ordered the Bilsteins today from RD, BofD sport part 
numbers. I'll measure the relative perches when they get here, see if I need 
collars/spacers.

Had a little bit of snow/slush today, nothing serious. quattro == glue.
Henry Harper
http://www.srv.net/~hah
1991 200 quattro, 80k, look, I'm driving it, keeping the aquatreads on to make it snow
1988 GTI 16v, 173k, can only drive one car at time, needs a rack boot anyway